In laboratory centrifuges, the rotor, i.e., the part of the structure which rotates and which carries a vessel with material to be subjected to centripetal force, is balanced at the time of manufacture. Nevertheless, in the event of a defect or of uneven loading of the vessel, an imbalance may arise that can be tolerated only within specific limits. Otherwise, damage may occur when operating the centrifuge, especially at high speeds.
Accordingly, centrifuges of this kind are equipped with shutoff devices for turning the motor off when an upper threshold imbalance, empirically ascertained for the particular centrifuge, is exceeded.
However, ascertaining the imbalance arising at centrifuge startup entails difficulties.
The state of the art comprises highly costly shutoff devices which typically operate using magnetic-field detectors to monitor the rotor-generated magnetic fields and to thereby determine the imbalance.
Known centrifuges have been marketed by applicant's assignee for many years in which a mechanical switch is mounted on the housing and, upon rotor imbalance and lateral deviation, the switch makes contact with an element mounted on the stator which then actuates the shutoff switch. This design, however, incurs two substantive drawbacks. On one hand, mechanical switches may fail per se and on the other hand the switch or the element on the stator must be adjusted to assure that switching off takes place accurately at the specific threshold imbalance. Assembly costs are raised as a result. Furthermore, the deviation depends on support tolerances and therefore will differ among units of the same type at the same imbalance.